This invention relates generally to percussion tools used in downhole drilling. More particularly, this invention relates to an apparatus and method for controlling air flow within percussion tools, such as rotary bits, shear bits, and lighter hammer bits, used in downhole drilling.
Rotary drilling tools, such as rock bits, can benefit from percussive energy to improve drilling rate, or rate of penetration (ROP), and improve hole straightness. However, this percussive energy should be controlled. If the percussive energy is too little, the drilling tool will not create and/or propagate fractures in the rock. If the percussive energy is too much, the drilling tool life is unacceptably reduced due to bearing spalling, steel fatigue cracking, and/or other life reducing causes. Hence, to be an effective tool, the drilling tool should be efficient with low drill system pressure, but also should be able to limit percussive force at high drill system pressure.
There are currently two rotary percussion systems in the market, the PARD tool and the RPS tool. However, neither of these tools are meeting market needs. The PARD tool provides sufficient percussive force at low drill system pressures, but vents excessive air through an unreliable top vent system which commonly plugs, thereby resulting in work stoppages. The RPS tool suffers from inadequate percussive force at low drill system pressures, but vents excessive air through a central flow system which is not prone to plugging. Low drill system pressures include pressures in the sixty pounds per square inch (psi) to eighty psi range, but can be different from the provided range. There is a need to develop a tool which achieves high percussive force at low drill system pressures while reliably limiting percussive force at higher drill system pressures to avoid damage to the rock bit.
FIG. 1A is a longitudinal cross-sectional view of a portion of a conventional downhole percussion tool 10 in accordance with the prior art. FIG. 1B is a longitudinal cross-sectional view of a remaining portion of the conventional downhole percussion tool 10 of FIG. 1A whereby FIG. 1A is intended to be joined to FIG. 1B along common line a-a in accordance with the prior art. The conventional downhole percussion tool 10 is described in detail in U.S. Pat. No. 7,377,338, which issued to Bassinger on May 27, 2008, and is incorporated by reference herein in its entirety. Thus, the conventional downhole percussion tool 10 is briefly described herein for the sake of describing airflow therein. Referring to FIGS. 1A and 1B, the conventional downhole percussion tool 10 includes a tool cylinder or housing 12, a rear adapter or sub 24, a check valve 36, a piston 44, a drive sub 106, and an integrated claw bit 92. Although an integrated claw bit is illustrated within FIG. 1B, a bit sub (not shown) capable of receiving a claw bit, or other bit type, can be used in lieu of the integrated claw bit 92. Once the conventional downhole percussion tool 10 is assembled, a top pressure fluid chamber 78, an annular chamber 97, and a bottom pressure fluid chamber 88 is formed.
The sub 24 includes a sub passage 30 extending longitudinally therein. The check valve 36 is coupled at an end of the sub passage 30 and is positioned within the housing 12 once the sub 24 is threadedly coupled to an end of the housing 12. The check valve 36 allows for pressurized fluid to flow from the sub passage 30 into the housing 12; however, the check valve 36 prevents pressurized fluid from flowing from the housing 12 to the sub passage 30.
Similarly, the drive sub 106 is threadedly coupled to an opposing end of the housing 12. The integrated claw bit 92 is movably coupled within the drive sub 106 at the opposing end of the housing 12. The integrated claw bit 92 includes a bit passage 118 extending longitudinally therein and is in communication with one or more secondary bit passages 120, which are in communication with an environment external to the bit 92. The integrated claw bit 92 is capable of moving in at least an axial direction and may be capable of moving in a rotational manner as well. When the integrated claw bit 92 is in contact with the bottom of the formation or when there is a significant upward force acting upon the integrated claw bit 92, the integrated claw bit 92 is in the dash-lined position as shown in FIG. 1B. Conversely, when the integrated claw bit 92 is not in contact with the bottom of the formation or there is no significant upward force acting upon the integrated claw bit 92, the integrated claw bit 92 is in the solid-lined position as shown in FIG. 1B.
The piston 44 is a single-walled tube that includes a piston passage 70 extending substantially centrally therethrough. An orifice plug 74, or choke valve, is positioned within the piston passage 70 at a top end of the piston 44. The piston passage 70 is in fluid communication with piston base passage 72 formed within an opposing end of the piston 44. The piston 44 also includes at least two pressurized fluid inlet ports 82 formed along a top portion of a sidewall of the piston 44 and extending into an interior of the piston 44. The piston 44 further includes pressurized fluid conducting piston passageways 80 extending from the pressurized fluid inlet ports 82 to the opposing end of the piston 44. Piston 44 further includes one or more exhaust passages 96 that extend from the piston base passage 72 to the annular chamber 97 formed between the piston 44 and the housing 12. The exhaust passages 96 are offset from the pressurized fluid conducting piston passageways 80. The piston 44 is movably positioned within the housing 12. Once the piston 44 is properly assembled within the housing 12, the top pressure fluid chamber 78, the annular chamber 97, and the bottom pressure fluid chamber 88 are formed. The top pressure fluid chamber 78 is formed between the one end of the piston 44 having the orifice plug 74 and the check valve 36. The annular chamber 97 is formed between a portion of the perimeter of the piston 44 and the housing 12. The bottom pressure fluid chamber 88 is formed between the opposing end of the piston 44 and the integrated claw bit 92.
During operation of the conventional downhole percussion tool 10, the tool 10 is placed in a position such that the bit 92 is urged upwardly to the position indicated by the dashed lines in FIG. 1B and the piston 44 will be urged to the position shown by the solid lines in FIGS. 1A and 1B. In this position, the flow of high pressure fluid from top pressure fluid chamber 78 to annular chamber 97 is terminated since a reduced diameter portion 56 of the piston 44 is in close fitting relationship with a sleeve 62 positioned within the housing 12 and about the perimeter of a portion of the piston 44. In this condition, pressure fluid is still communicated through pressurized fluid conducting piston passageways 80 to bottom pressure fluid chamber 88 while pressure fluid is vented from annular chamber 97 through exhaust passages 96 to the exterior of the tool 10 by way of the bit passage 118 and secondary bit passages 120. Thus, a resultant force is exerted on the piston 44 driving it upwardly, viewing FIGS. 1A and 1B, until the reduced diameter portion 56a of the piston 44 is positioned such that the communication of high pressure fluid to pressurized fluid inlet ports 82, pressurized fluid conducting piston passageways 80, and bottom pressure fluid chamber 88 is cut-off. A resultant pressure fluid force acting on piston 44 will continue to drive the piston 44 upwardly, viewing FIGS. 1A and 1B, until the pressure fluid from bottom pressure fluid chamber 88 is able to vent through bit passage 118 and secondary bit passages 120. This occurs when the bottom of the piston 44 is raised elevationally above the top of a tube 124, which is positioned at least partially within bit passage 118 and extends outwardly from the top of the bit 92. In this condition, a net resultant pressure fluid force acting on the top surface of the piston 44 is sufficient to drive the piston 44 downwardly to deliver an impact blow to the top surface of the bit 92 and the cycle just described will then repeat itself rapidly and in accordance with the design parameters of the tool 10.
As seen in FIGS. 1A and 1B along with the description provided, it can be seen that the pressurized fluid is flowing into both the top pressure fluid chamber 78 and the bottom pressure fluid chamber 88 when driving the piston 44 in an upward direction. The pressure in both the top pressure fluid chamber 78 and the bottom pressure fluid chamber 88 are equal; however, since the area of the piston 44 adjacent the top pressure fluid chamber 78 is smaller than the area of the piston 44 adjacent the bottom pressure fluid chamber 88, there is a higher force acting upon the bottom surface of the piston 44 causing the piston 44 to move in an upward direction. This becomes very inefficient because the pressures within the top pressure fluid chamber 78 and the bottom pressure fluid chamber 88 are being used against one another.
The drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments.